Levels, sources, and risk assessment of PAHs residues in soil and plants in urban parks of Northwest China

Polycyclic aromatic hydrocarbons (PAHs) will be ingested by people through different ways to threaten their health during play, so the environmental quality of the park directly affects the health of tourists and residents. Using eight typical parks in Urumqi in Northwest China as the study area, we used GC–MS to detect the PAHs content in the park surface soil and 10 common plants in the park in different seasons. The results showed that the content of PAHs in park soil in the summer was 5–6 times that in the winter, and the monomer PAHs in some park soil sampling points were higher than the soil pollution risk screening value. And the contamination level at these sampling sites was also higher compared to other sampling sites. In summer, the plants with high PAHs content in leaves are short herbs, while in winter, they are tall arbors. The PAHs of the park soil are mainly composed of high-cyclic aromatic hydrocarbons, and are mainly of traffic origin. The proportion of low-ring aromatic hydrocarbons in the winter was significantly higher than that in the summer. The source of PAHs in plants in summer is similar to that in soil, but the source of PAHs in plants in winter is more complex. The toxicity equivalent concentration method values of soil PAHs in South Park, Zhiwu Park, Shihua Park and Toutunhe Park were higher than that in other parks. The lifetime carcinogenic risk (ILCRs) values of some sampling points in these four parks in the summer were relatively high. The average ILCRs of adults and children in all parks reached a low-risk level in summer. The carcinogenic risk in children is much higher than that of adults.

standard for construction land (GB36600-2018), the value of BaP at sampling point 3 of Zhiwu Park in the soil of the park in summer is 0.944 mg/kg, which is greater than the screening value of 0.55 mg/kg. The values of DahN at sampling point 3 of Zhiwu Park and sampling point 3 of Toutunhe Park are 1.469 mg/kg and 0.615 mg/kg, respectively, which are greater than the screening value of 0.55 mg/kg, whereas BAP and DahN mainly originate from vehicle exhaust emissions, such as gasoline and diesel combustion 12 . This shows that the traffic congestion near Zhiwu Park and Toutunhe Park in summer needs to be reasonably controlled. The concentrations of PAHs in other soil sampling points in the summer and winter were lower than the screening value for soil pollution risk.
PAH-contaminated soil can be divided into four levels: no pollution (< 0.200 mg/kg), slight pollution (0.200-0.600 mg/kg), medium pollution (0.600-1.000 mg/kg) and serious pollution (> 1.000 mg/kg) 13 . As we can see from  14 . Among the soil samples in the summer, 91.6% were more than moderately polluted, and only 8.4% were pollution-free. This is because the park has a large passenger flow, especially in the summer, and it is located adjacent to the main traffic road. Traffic volume is large, and traffic emissions are one of the main sources of urban PAHs 15 . In the soil of the winter park, the sampling point No. 1 of South Park and sampling point No. 2 of Shihua Park were moderately polluted. The total concentration of PAHs in the three sampling points of Toutun Park was greater than 1.000 mg/kg, reaching the level of serious pollution. The most polluted point was the sampling point No. 3. Some other sampling points were slightly polluted, and some were pollution-free. Toutun Park is located near the Diwopu Airport. Due to the re-planning and reconstruction of the park in recent years, the exhaust emissions of large trucks and the wear of tires during the reconstruction process have greatly increased the accumulation of PAHs in the park 16 . Shihua Park is located near Midong District. There are numerous chemical plants of petrochemical companies, plastic factories, which generate a large number of PAHs during the production and processing of products. The South Park is located in a downtown area with heavy traffic and many nearby residential areas. Compared with other parks, there are more man-made emission sources near these three parks. In winter, 58.3% of soil samples were polluted, of which 20.8% were moderately polluted or above, and 41.7% were pollution-free. Compared with the summer, the proportion of contaminated samples decreased.
As shown in Table 2, the average order of the soil PAHs in each functional area in summer is traffic area > industrial area > park area > rural area > commercial area, which is consistent with the research by Min et al. 17 . The park is located adjacent to the main traffic road. Compared with the rural and commercial areas, the park area has a larger passenger and vehicular traffic flow. Compared with the traffic area, the traffic volume in the park area is small, and industrial activities such as coke oven gas, organic chemical industry, petroleum industry, steelmaking, and ironmaking in the industrial area generate a large amount of PAH emissions 17 . Conversely, plant leaves in the park absorb PAHs from atmospheric deposition and accumulate PAHs, such that the PAHs entering the soil of the park through atmospheric deposition is greatly reduced. Compared with other regions, the content of soil PAHs in the Urumqi park area in the winter is similar to that in Beijing and Xi'an park area, which is slightly higher than that in the Yinchuan and Fuzhou park areas. This is because Urumqi, Beijing and Xi'an have a large population, and heating in the winter generates more PAHs. The average value of PAHs in the summer park area was 5-6 times that in winter. This is consistent with the results of Fangfang et al. 18 19 . This also includes the city of Urumqi. Outdoor activities were not resumed until September 2020, when the outbreak was controlled in Urumqi. And the restriction of outdoor activities potentially reduces the accumulation of PAHs in the soil in winter 2020. Overall, the PAH pollution of soil in the park area of Urumqi City is relatively serious and should be considered. The PAH content of plants in the South Park of Urumqi is shown in Table 1. Comparing plant leaves and roots, we can see that whether it is summer or winter, the proportion of Σ 7 PAHs in Σ 16 PAHs is always higher than that of leaves. This is because Σ 7 PAHs are mainly composed of 4-6 ring aromatic hydrocarbons, and the absorption of roots from soil is the main absorption pathway of high-ring PAHs. Low-ring PAHs are the result of the combined effect of leaf absorption from the atmosphere and root absorption from the soil 20 . In addition, the proportion of Σ 7 PAHs in plant leaves in the winter to the concentration of Σ 16 PAHs is considerably different from that in plant leaves and roots in summer and roots in winter. This is because plant leaves mainly absorb PAHs in the atmosphere 21 , and PAHs in the atmosphere are more susceptible to the impact of surrounding emission source than soil. Due to the control of the epidemic, vehicle transportation restrictions led to a reduction in the proportion of high-ring PAHs in the winter.
As shown in Fig. 1B, the highest total concentration of PAHs in the leaves of park plants in summer was Elm leaves, followed by Rosa acicularis leaves and Artemisia annua leaves. These herbaceous plants are relatively short and grow close to the ground, as a result the content of PAHs in their leaves is greatly affected by ground dust 22 . The highest total concentration of PAHs in plant leaves in winter parks is in Juniper leaves, followed by www.nature.com/scientificreports/  www.nature.com/scientificreports/ Platycladus leaves and Dragon claw willow leaves. Because these plants belong to arbor plants, the plants are relatively high and the leaves are located far away from the ground, the PAHs content in the leaves is easily affected by PAHs in the atmosphere. In winter, due to heating, the combustion of coal and biomass mainly produces tricyclic PAHs, which are volatile and easily transferred to the atmosphere 23 . In addition, Juniper and Platycladus are evergreen plants in all seasons, and their leaves do not easily fall off. Compared with other plants, they are more likely to accumulate PAHs. Pine needles have a large specific surface area, high surface wax content, wide distribution, and easy collection. They have been used by many foreign environmental scientists to monitor and evaluate organic pollutants in the environment 24 . We can also observe from Fig. 1C that the total concentration of PAHs in 70% of the plant leaf samples was greater in the winter than in the summer. This is owing to volatilization through stomata caused by the rising temperature in the summer. PAHs mainly migrate from leaves to the atmosphere 25 . The content of PAHs in the roots of Clover and Silver edge grass was higher in different seasons.
It is speculated that this may be related to the structure of the plant. These two herbaceous plants may be used to repair soil pollution caused by PAHs in the soil 26 . In the study by Yuanyuan et al. 10 , it was found that legume alfalfa and gramineous perennial ryegrass exhibited a higher removal rate of soil PAHs, which was mainly because plants promoted the joint degradation of PAHs in soil by indigenous microorganisms 10 . Table 2 shows a comparison of the PAH content of plants in this study and other regions. The range of total PAHs in summer plants in Urumqi parks is 0.033-3.941 mg/kg, which is higher than the change range of PAHs in plants considered by Dong Ruibin and others in the range of 0.020-1.000 mg/kg 26 . The concentration value is close to the Nanjing campus area and Guangzhou scenic area, but much lower than the Changsha traffic area. Studies have shown that when a car is driving at a non-uniform speed, the exhaust contains more PAH pollutants, and the PAH content in the exhaust particulate matter also increase relatively 27 . In addition, our results were slightly higher than that in Qingdao agricultural area. This is because the research conducted by Wei et al. 27 is farmland soil, which is located at the edge of the city and is far away from the city center. Although there are pollution sources such as industry and traffic around some sampling points, these are relatively limited, such that the content of PAHs in plants is close to other regions with relatively low urbanization and industrialization. The average total PAH content of plants in the park area in the summer was twice that in the winter, which is consistent with the PAH content in the soil. This may be correlated with climatic conditions and control of the epidemic.
PAHs monomer component characteristics. According to the number of rings in PAHs, the 16 types of PAHs are divided into 2-3 low-ring aromatic hydrocarbons (LMW) and 4-6 high-cyclic aromatic hydrocarbons (HMW) 12 . Among them, the 2-ring PAHs are mainly derived from the leakage of source oil, while the 3-ring and 4-ring PAHs are mainly derived from the combustion of coal and biomass, the 5-ring PAHs are mainly derived from the combustion of gasoline, and the 6-ring PAHs are mainly derived from the combustion of diesel.
As shown in Fig. 2, BbF, BkF, InP, and DahN accounted for a higher proportion in the summer soil. BbF mainly originates from the combustion of gasoline, BkF mainly originates from industrial coal, and InP and DahN mainly originate from automobile exhaust 12,28 . The order of the proportion of the number of different PAHs was 5-ring PAHs > 4-ring PAHs > 6-ring PAHs > 3-ring PAHs > 2-ring PAHs. It is speculated that the main source of PAHs in the summer park soil may have originated from gasoline combustion and traffic emissions 16 .
Fla, Phe, Pyr, and BbF accounted for a higher proportion of the soil PAHs in the winter. Fla mainly originates from the combustion of coal and petroleum and can also originate from garbage incineration. Phe mainly originates from the process of coking, which is the process of decomposing and distilling coal to produce coke, and Pyr mainly originates from industrial coal 29 . The order of the proportions of the number of different PAHs was 4-ring PAHs > 5-ring PAHs > 3-ring PAHs > 6-ring PAHs > 2-ring PAHs. Compared with the summer, the proportion of the 3rd and 4th rings in the winter has increased significantly, and the 3rd and 4th rings of PAHs are mainly derived from the combustion of coal and biomass, which is caused by heating in the winter. In addition, it can be found that the contribution of HMW is higher in both summer and winter, which may be owing to the volatility of LMW 23 .
We can observe from Fig. 2 that the higher proportions of summer plants were BbF, Phe, InP, and BkF. Barring Phe, the main proportion of PAHs was similar to the proportion of PAHs in summer park soil, indicating that the proportion of different PAHs in plants is greatly affected by soil. Phe accounts for a relatively small proportion in the summer soil, but a larger proportion in plants. This is because, due to its small molecular weight, Phe is easily volatilized from the soil when the temperature is high in the summer and is thereby absorbed by plant leaves 30 . The order of the proportion of the number of different PAHs was 5-ring PAHs > 3-ring PAHs > 4-ring PAHs > 6-ring PAHs > 2-ring PAHs. Similar to summer soils, high-ring PAHs accounted for a higher proportion, but compared with summer soils, the proportion of LMW increased. This is because low-molecular-weight PAHs have 2-3 rings, which are mainly gaseous and are easily absorbed by plants.
Phe, NaP, Flu, and Fla accounted for a higher proportion of plants in the winter. Among them, Flu mainly originates from coking, and NaP mainly originates from coke oven products and oil spills 30 . The order of the proportion of the number of different PAHs was 3 ring PAHs > 4 ring PAHs > 5 ring PAHs > 2 ring PAHs > 6 ring PAHs. Compared with the summer, the proportion of low-ring PAHs in winter plants and soil has increased significantly, which may be caused by the burning of large amounts of coal and organisms for heating in winter in the north. In winter, the proportion of HMW in plant roots and leaves is different, and the proportion of HMW in root is higher than that in leaves. This is because the roots are underground, and it is easier to absorb high-ring PAHs from the soil than leaves. In addition, the proportion of LMW in plants in both winter and summer was higher than that in the soil. This is because foliar absorption is the main mechanism by which plants accumulate PAHs 31 . www.nature.com/scientificreports/ PAHs source analysis. The feature ratio method is often used to identify the source of PAHs in the environment 32,33 . LMW/HMW > 1 represents the crude oil source, LMW/HMW < 1 represents high-temperature pyrolysis; Ant/(Ant + Phe) < 0.1 represents the crude oil source, Ant/(Ant + Phe) > 0.1 represents the combustion source; BaP/BghiP ratio > 0.6 means traffic emission source, less than 0.6 means non-traffic emission; BaA/(BaA + Chy) < 0.2 characterizes crude oil source; 0.2 < (BaA/BaA + Chy) < 0.35 is used to characterize the combustion of coal, vegetation, etc.; finally (BaA/BaA + Chy) > 0.35 is usually used to characterize the combustion of petroleum and fossil fuels 32,34 . As shown in Fig. 3, the ratio of LMW/HMW in summer soil was less than 1, 54.2% of the Ant/(Ant + Phe) ratio was greater than 0.1, and 91.7% of the BaP/BghiP ratio was greater than 0.6. All BaA/(BaA + Chy) ratios were greater than 0.35. We can infer from this that the PAHs in the soil of Urumqi City Park in the summer mainly originate from traffic emissions, that is, the combustion of crude oil fuels such as gasoline and diesel.
In winter, 79.2% of the LMW/HMW ratio in the soil was less than 1, 95.8% of the Ant/(Ant + Phe) ratio was greater than 0.1, the BaP/BghiP ratio was greater than 0.6, and 75.0% of the BaA/(BaA + Chy) ratio was between 0.2 and 0.35. We can infer from this that the PAHs in the soil of Urumqi city parks in winter mainly originate from high-temperature sources, including incomplete combustion of coal, grass, and other biomass and fossil fuels.
As shown in Fig. 4. The LMW/HMW ratios of summer plants were all less than 1, 93.3% of the Ant/(Ant + Phe) ratios were less than 0.1, 73.3% of the BaP/BghiP ratios were greater than 0.6, and 73.3% of the BaA/(BaA + Chy) ratios were greater than 0.35. From this, we can infer that the PAHs in plants of Urumqi park in the summer are similar to those in the soil, and they mainly originate from traffic emissions.
In winter plants, 66.6% of the LMW/HMW ratios were greater than 1, all the ratios of Ant/(Ant + Phe) were less than 0.1, 86.6% of the BaP/BghiP ratios were greater than 0.6, and 33.3% of the BaA/(BaA + Chy) rations were less than 0.2, 33.3% were between 0.2 and 0.35, 33.3% were greater than 0.35. We can infer from this that the source of PAHs in park plants in Urumqi in winter is more complicated, and the leakage of crude oil, burning of coal biomass, and traffic emissions all contribute to a certain extent. Table 3 displays the toxic equivalent concentrations of PAHs in the soil of parks in Urumqi. The TEQ range of PAHs in the summer soil of the park was 0.024-2.937 mg/kg, with an average of 0.733 mg/kg, while the average TEQ of Σ 7 PAHs was 0.729 mg/kg, accounting for 99.5% of the Σ 16 PAHs, and the TEQ of the remaining PAHs only accounts for 0.5%, which shows that Σ 7 PAHs are the primary contributing factors to the toxic effect of the PAHs in the summer soil of the park. The contributions of different monomer PAHs to the total TEQ were as follows: DahN (42.8%) > BaP (35.6%) > BbF (7.5%) > InP (6.7%) > BkF (4.3%) > BaA (2.8%) > Chy (0.3%). Further, 25% of the total TEQ value of PAHs in the soil in Urumqi Summer Park is higher 6  www.nature.com/scientificreports/ than the standard value of 1 mg/kg given by the World Health Organization (WHO) 35 . These sampling point mainly originate from South Park, Zhiwu Park, Shihua Park and Toutunhe Park. Therefore, direct or indirect contact with the soil of these four parks in the summer cause a certain risk of toxic effects to humans. The TEQ range of PAHs in the winter soil was 0.003-0.656 mg/kg, with an average value of 0.096 mg/kg; the average TEQ of Σ 7 PAHs was 0.095 mg/kg, accounting for 99.0% of the Σ 16 PAHs, and the TEQ of other PAHs only accounts for 1.0%, and similar to summer soil, Σ 7 PAHs are almost all contributing factors to the toxic effect of PAHs in the winter soil. The contribution of different monomer PAHs to the total TEQ was in the following order: BaP (41.8%) > DahN (37.8%) > BbF (7.2%) > BaA (4.6%) > InP (4.2%) > BkF (2.7%) > Chy (0.7%). The total TEQ value of PAHs in the soil in Urumqi winter is lower than the standard value of 1 mg/kg given by the WHO. Therefore, direct or indirect contact with Urumqi parks soil in winter will not cause toxic effects to humans.

PAHs risk assessment.
The lifetime carcinogenic risk (ILCRs) less than or equal to 10 -6 are considered to be basically negligible lifetime cancer risk; ILCRs between 10 -6 and 10 -4 indicate low risk; ILCRs greater than 10 -4 indicate high potential health risks 36 .
As shown in Fig. 5, the mean values of ILCRs for adults and children regarding PAHs in the summer soil were 2.783 × 10 -6 and 3.400 × 10 -6 , respectively. ILCRs in adult, 83.3% were greater than 10 -6 , and in children, 87.5% were greater than 10 -6 , reaching a low risk level. Among them, the maximum ILCRs in adults and children were greater than 10 -5 at sampling point 3 of the Botanical Garden. The mean values of ILCRs for adults and children regarding PAHs in the winter soil were 3.632 × 10 -7 and 4.439 × 10 -7 , respectively, which were lower than the minimum ILCRs. Only 12.5% of adult and child ILCR values greater than 10 -6 reached a low risk level, and the sampling points were all in Toutunhe Park. In addition, we can observe that the carcinogenic risk of soil in the park is much higher in children than in adults. This is because children's organs, nerves and immune systems may be more sensitive to pollutants, as children are developing; at the same time, children move easily on the ground and have greater access to contaminated soil. The carcinogenic risk of the summer soil to the human body was much higher than that of winter soil. This is inconsistent with the results of Fangfang et al. 18 . The reason may be that Urumqi is affected by the new coronavirus (COVID- 19), and the amount of artificially produced PAHs decreased. The ILCRs in adults and children in both summer and winter are as follows: ingestion > dermal > inhalation. Compared with other exposure routes, the lifetime carcinogenic risk of inhalation is nearly negligible. This is because the lifetime carcinogenic risk caused by children's frequent hand-to-mouth contact while playing in the park is much higher than that of inhalation. The results show that the ILCRs of the PAH exposure pathway of soil samples from Urumqi parks in winter were all lower than the minimum acceptable risk level. The lifetime carcinogenic risk of adults and children is negligible, but high ILCRs in summer require particular attention.    InP 0.1 2.3 × 10 -2 to 7.6 × 10 -2 4.9 × 10 -2 7.9 × 10 -5 to 2.9 × 10 -2 3.9 × 10 -3 www.nature.com/scientificreports/ The NIPI characterizes the impact or effect of soil pollutants. NIPI can be divided into five levels: NIPI is less than or equal to 0.7, where soil pollution is at a safe level; NIPI is equal to 1.0, where soil pollution is at the warning line; NIPI between 1.0 and 2.0, where soil pollution is at a weak level; NIPI between 2.0 and 3.0, where soil pollution is at a moderate level; and NIPI greater than 3.0, where soil pollution is severe 37 .
As shown in Fig. 6, in summer, the NIPI range of park soil in Urumqi is 0.025-3.532, with an average value of 0.715. The NIPI value of 75% of the sampling points was less than 1.0, and the ecological risk of the soil was safe. The NIPI value of 20.8% of the sampling points is between 1.0 and 2.0, mainly located in South Park, Zhiwu Park, Shihua Park and Toutunhe Park. The soils of these parks have more PAH accumulation factors than the soils of other parks. At the same time, the maximum value of NIPI is 3.532, which is a severe pollution level. This may be related to the pile of garbage in the park. The NIPI range of soil in Urumqi city parks in winter was 0.004-0.564, with an average value of 0.095. The NIPI value of all sampling points was less than 0.7, indicating that the ecological risk of soil in all parks in winter is at a safe level. The ecological risk of park soil in Urumqi in winter is much lower than that in summer.
In general, the ecological risks of soil in Urumqi parks can be ignored. Among them, South Park, Zhiwu Park, Shihua Park and Toutunhe Park should adopt a variety of control measures to reduce the ecological risk of soil PAHs through strict environmental management.

Conclusion
In eight typical parks in Urumqi, the average concentration of PAHs in soil in the summer was 3.304 mg/kg. The average concentration of PAHs in soil in the winter was 0.644 mg/kg. The single PAHs in some soil sampling points in summer were higher than the national "Construction Land Standard" (GB36600-2018) soil pollution  www.nature.com/scientificreports/ risk screening value. In summer, 91.6% of the soil samples were moderately or severely polluted, while 20.8% of the soil samples in winter were moderately or severely polluted. The average value of PAHs in soil in the park area in summer was 5-6 times that in winter. PAHs are mainly composed of high-ring PAHs from traffic sources, but the proportion of low-ring PAHs 2-3 increases significantly in winter. Compared with other regions, pollution is more serious, especially in the summer. In summer, the plants with high PAHs content in leaves are short herbs, while in winter, they are tall arbors. The proportion of seven carcinogenic PAHs was higher in the roots than in the leaves. PAHs in 70% of plant leaf samples were higher in winter than in summer. PAHs are mainly composed of five rings in summer, from traffic sources, and three rings in winter, from the combustion of coal biomass. Compared with plants of other areas, pollution is lighter.
The TEQ values of soil PAHs in South Park, Zhiwu Park, Shihua Park and Toutunhe Park were higher. The summer soil of these four parks is at a low pollution level, compared with other parks. The mean ILCRs in adults and children in all parks reached a low-risk level in summer. The cancer risk of children in different seasons is much higher than that in adults. The sequence of ILCRs for different exposure routes is ingestion > dermal > inhalation, and the lifetime cancer risk of inhalation can be almost ignored.

Materials and methods
Overview of the study area and sample collection. In this study, eight typical parks in Urumqi City were selected as the soil sampling sites. As shown in Fig. 7, these are: South Park, Shuimogou Park, Xiaolügu Park, Renming Park, Zhiwu Park, Shihua Park, Hongguangshan Park and Toutunhe Park. The sample collection periods were from November 20 to November 22, 2020 (winter), and from June 13 to June 14, 2021 (summer). According to the area of the park, nine sampling points were selected for each park, and every third soil samples were mixed into a mixed sample. The depth of sampling was 0-10 cm. In total, 48 surface soil samples were collected. Meanwhile, South Park was selected for the sampling of plants because it has a large number of plant species and higher passenger flow. Ten kinds of common plants in the park were selected, including five kinds  Sample handling. We accurately weighed 15 g (accurate to 0.01 g) of soil samples and 1 g of plant samples after freeze-drying, grinding, and screening, put them into 250 ml and 50 ml centrifuge tubes, added 30 ml and 20 ml (v:v = 1/1) dichloromethane acetone solution, respectively, and allowed to stand for 2 h. Homogenates were extracted on the Beater (ULTRA-TURRAX) for 1 min, and then extracted in a water bath constant temperature (30 °C) oscillator (SHA-C) for 30 min. Subsequently, the mixture was centrifuged using a centrifuge (CT18RT) at a speed of 10,000 r/min for 5 min, and then 5 ml of upper organic phase clear liquid was taken. The mixture was concentrated at 40 °C with a parallel evaporator (BUCHI-Syncore), and the volume was combined with 2.5 ml of n-hexane, swirled for 15 s, and then passed through a 0.22 μm membrane injection bottle for testing.
Test analysis. GC Toxicity equivalent concentration method (TEQ). The carcinogenicity and toxicity of benzo(a)pyrene (BaP) were the strongest among the seven carcinogenic PAHs. Usually, BaP is used to evaluate carcinogenic PAHs, and the toxic effect factor (TEF) is a parameter describing the carcinogenic ability of each monomeric PAH corresponding to BaP [38][39][40] . The formula for calculating the equivalent concentration of PAHs (TEQ) based on the toxic equivalent factor is as follows 40,41 : where PAH i is the content of PAH monomer i, TEF i is the toxicity equivalent factor of PAH monomer i, TEQ is the toxicity equivalent of the equivalent concentration of the compound, and the TEF value of BaP is 1, which is the highest among all PAHs 42 . According to the toxicity equivalent of BaP, we used the toxicity equivalence factor (TEF) shown in Table 4 to convert the concentration of PAHs into the toxicity equivalent concentration of PAHs.

Incremental Lifetime Carcinogenic Risks (ILCRs). The incremental lifetime carcinogenic risk (ILCRs)
of soil PAH exposure was evaluated according to USEPA standards 43 . Children and adults are exposed through three main exposure routes: ingestion, dermal and inhalation 44 . The ILCR equation for soil is as follows: (1) TEQ = (PAH i × TEF i ) www.nature.com/scientificreports/ where CS is the toxic equivalent concentration of monomeric PAHs in the sample (mg/kg), 10 6 is the conversion coefficient of PAH concentration, and CSF is the carcinogenic slope factor (mg/(kg·d)), based on the carcinogenic ability of BaP. The CSF Dermal , CSF Ingestion , and CSF Inhalation are 25, 7.3, and 3.85 (mg/(kg·d)), respectively 44 . Other parameter values were obtained from the Guidelines for Site Environmental Assessment (DB11/T 656-2009), as shown in Table 4.

Nemerow multifactor pollution index (NIPI). The Nemerow multifactor pollution index (NIPI) is one
of the most widely used indicators to reflect the ecological risk of soil pollutant exposure 37 . It can characterize the role of soil pollutants in a seriously polluted environment. The calculation method for the NIPI is as follows: where C i is the measured concentration of monomer i, C s is the soil environmental quality standard value of monomer i, and Pi is the pollution index of PAH monomer i in the soil. P iavg is the arithmetic mean of PAH pollution index of each monomer, P imax is the maximum value of the PAH single factor pollution index at the ith sampling point, and NIPI is the multi-factor pollution index at the sampling point. This study used the class II standard value in the soil environmental quality standard (GB15618-2008). The use of plants in the present study complies with international, national, and institutional guidelines, the collection of plant samples was done with the consent of local relevant departments and institutions.